![]() Sound waves can only propagate in material medium (in gases, liquids, solids, plasmas etc. Light is an electromagnetic wave, and as for any wave its velocity is determined by the *medium* in which its propagating. For most calculations the value 3.00 x 10 5 km/s is used.Is it possible that the speed of light is different on other planets, like how on other planets, the speed of sound could be affected by the amount of particles in the atmosphere? It is always slower in other materials such as water or glass. As a result, the numerical value of the speed of light ( c) in meters per second is now fixed exactly by the definition of the meter. Then, in 1983, the meter was redefined in the International System of Units (SI) as the distance traveled by light in vacuum in 1/299,792,458 of a second. ![]() In the 1970s, interferometry was used to get the most accurate value for the speed of light that had been measured yet: 299,792.4562☐.0011 km/s. For the first time the speed of light could be measured on Earth, and the speed of light was measured to very great accuracy. This method was based on a similar apparatus built by Armand-Hippolyte Fizeau. In the 1850s, French physicist Jean Foucault measured the speed of light in a laboratory using a light source, a rapidly rotating mirror and a stationary mirror. This is only about 0.5% off the modern known value of the speed of light. But using his method with the knowledge of distances we have today, we get a value of approximately 301,204.8 km/s for the speed of light. He didn't actually calculate the speed of light as the diameter of Earth's orbit was not well known in his day. He interpreted this to be the amount of time it takes light to travel across the diameter of Earth's orbit. Rømer found the maximum variation in timing of these eclipses to be 16.6 minutes. He noticed that when Jupiter and Earth are far apart (near conjunction), the eclipses of the moons occurred several minutes later than when Jupiter and the Earth are closer (near opposition.) He reasoned that this could be because of the time light takes to travel from Jupiter to Earth. In 1676 a Danish astronomer named Ole Rømer was studying the orbits of the moons of Jupiter and making tables to predict when eclipses of the moons would occur. It is understandable that Galileo was unable to measure this with his pulse! We now know the speed of light very precisely, and if Galileo and his assistant were on hilltops one mile apart, light would take 0.0000054 seconds to travel from one person to the other. Galileo concluded that the speed of light was too fast to be measured by this method, and he was correct. He and his assistant tried this with different distances between them, but no matter how far apart they were, he could measure no difference in the amount of time it took the light to travel. Using the distance between the hilltops and his pulse as a timer, Galileo planned to measure the speed of light. Galileo and an assistant each stood on a different hilltop with a known distance between them, the plan was for Galileo to open the shutter of a lamp and then for his assistant to open the shutter of a lamp as soon as he saw the light from Galileo's. ![]() Galileo Galilei was the first person to attempt to measure the speed of light, in the early 1600s. Today anyone can use Google to search for the speed of light in a vaccuum and get an accurate result in seconds: 299 792 458 m/s. But who discovered the speed of light, and how did they do it?
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